1. Field of the Invention
The present invention relates to an electronic switching system having on the extension side an ISDN interface for connecting terminals to the ISDN, and more particularly to an apparatus and a method of setting a reason of disconnecting a communication line when the terminals connected to the ISDN interface do not respond to an incoming call to the ISDN interface.
2. Description of the Related Art
With ISDN now being widely used, electronic switching systems capable of being connected to the ISDN have been developed.
One example of such electronic switching systems is provided with ISDN interfaces both on the ISDN side and the extension side. A plurality of ISDN terminals can be connected to each of the ISDN interfaces on the extension side.
In the electronic switching system of this type, when there is an incoming call to an extension-side ISDN interface, the switching system sends a call setting message to an ISDN terminal connected to the extension-side ISDN interface to inform the terminal of the arrival of the incoming call in accordance with the procedures of the ISDN protocol.
Then, the electronic switching system monitors the generation of a response signal to the call setting message at the called ISDN terminal.
When no response signal is generated at the called ISDN terminal within a predetermined time, the electronic switching system outputs a message including a disconnecting reason indicating that there is no response to the calling terminal.
When non-coincidence of attributes is indicated by the called ISDN terminal, the switching system outputs to the calling terminal a message including the reason indicating that attributes of the terminals do not coincide with each other.
According to the above-described disconnection message setting, when an ISDN terminal is under communication and a calling terminal having the same attributes as the ISDN terminal under communication places a new call to an ISDN interface to which the ISDN terminal is connected, the terminal under communication cannot respond to the new call.
Therefore, the electronic switching system outputs to the calling terminal a message to the effect that there is no response, which correctly indicates the situation on the called side.
However, when a plurality of ISDN terminals are connected to the extension-side ISDN interface and when there is an incoming call to one of the ISDN terminals which does not respond to the incoming call, another one of the ISDN terminals may instead respond to that call.
Under that situation, if another ISDN terminal which has a different attribute from the first one of the ISDN terminals responds to an incoming call and sends to the interface a disconnection signal indicative of non-coincidence of attributes because the first terminal cannot respond to the call, the switching system inform the calling terminal of a disconnection reason indicating that the attribute of the calling terminal does not coincide with that of the called terminal.
Although there are some terminals which have the same attribute as the calling terminal in the extension-side ISDN interface and the system can respond to the calling terminal when the on-going communication is over, a wrong message indicative of "non-coincidence of attributes" is sent to the calling terminal. As a result, a user of the calling terminal may think that calling to the terminal is useless.
As described above, in the conventional electronic switching system in which a plurality of ISDN terminals are connected to an extension-side ISDN interface, when an appropriate ISDN terminal cannot correctly respond to an incoming call to the extension-side ISDN interface, a reason for disconnection is created on the basis of the response by the called ISDN terminal and is sent to the calling terminal.
Therefore, if there is another communication channel in the ISDN interface through which a terminal is under communication whose attribute is the same as that of the calling terminal, it is impossible to correctly inform the calling terminal that the reason that it could not respond to the incoming call was because the terminal having the same attribute as the calling terminal is under communication.
When the terminal under communication having the same attribute as the calling terminal completes its communication, it can respond to a retry by the calling terminal. However, the calling terminal may not redial because of the wrong message of "non-coincidence of attributes" and as a result, the reliability of the communication using the switching system is impaired.